Flash memory is one type of the nonvolatile semiconductor memory device. The transistor technology for the flash memory is generally divided into a floating gate technology and a MONOS (Metal-Oxide-Nitride-Oxide-Silicon) technology. The floating gate technology uses a transistor including a gate electrode with a double layer structure where a control gate is laid on a floating gate. In a floating-gate-type flash memory, the capacitive coupling between floating gates adjacent to each other increases as the memory is downsized, and therefore, further scaling is difficult. Accordingly, the MONOS technology without gate-to-gate coupling has become of interest.
The MONOS technology uses a transistor including a gate with a MONOS (metal/oxide film/nitride film/oxide film/silicon) structure. An electric charge is injected into and held at a trapping level present in this oxide film/nitride film/oxide film structure. Flash memories using this MONOS technology are disclosed for example in Non-Patent Documents 1 to 3.
Non-Patent Documents 1 and 2 disclose a charge trapping memory cell with a TANOS (TaN—Al2O3—SiN-Oxide-Si) structure where TaN is used as a material for the gate and Al2O3 is used as a material for the top insulating layer. Non-Patent Document 3 discloses a BE (Bandgap Engineered)-SONOS structure using a multilevel structure of an oxide film, a nitride film and an oxide film as a tunnel insulating layer.
In a memory cell of any of Non-Patent Documents 1 to 3, write and erase operations are performed by allowing an electric charge to pass through the bottom insulating layer located under a charge storage layer by means of the tunneling phenomenon.
Non-Patent Document 1: Chang-Hyun Lee et al. (Samsung), “Charge Trapping Memory Cell of TANOS (Si-Oxide-SiN—Al2O3—TaN) Structure Compatible to Conventional NAND Flash Memory,” IEEE Nonvolatile Semiconductor Memory Workshop 2006, pp. 54-55
Non-Patent Document 2: Chang-Hyun Lee et al. (Samsung), “A Novel SONOS Structure of SiO2/SiN/Al2O3 with TaN Metal Gate for Multi-Giga Bit Flash Memories,” IEDM 2003 Tech. Digest, pp. 613-616, December 2003
Non-Patent Document 3: Hang-Ting Lue et al. (Macronix), “BE-SONOS: A Bandgap Engineered SONOS with Excellent Performance and Reliability,” IEDM 2005 Tech. Digest, pp. 555-558, December 2005